Cellulose foams for high performance insulations

ABSTRACT

Environmentally friendly, sustainable, and high-performance ultralight composite foams are disclosed. The composite foams are prepared from cellulose nanomaterial, polymeric material, and a crosslinking agent. The fabrication process is simple and uses only water. The composite foams exhibit an elastic strain exceeding the values reported for known nanocellulose-based foams with no reinforcement. The foams exhibit a thermal conductivity superior to that of traditional insulating materials and retain structural integrity after burning.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/861,936, filed Jun. 14, 2019; the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Energy saving is a big challenge in the world, and any reduction in theenergy consumption through daily activities is considered a greatprogress. Thermally insulative materials play a vital role in energypreservation in buildings and construction, transportation, andpackaging industries. Expanded polystyrene has been widely used forcommercial lightweight, thermal insulation applications. However,expanded polystyrene is petroleum-based, composed of toxic styreneblocks, and exhibit relatively poor fire resistance with substantialtail ash floating in the air after burning. It therefore poses the riskof environmental contamination and health side effects. In addition,polystyrene does not lend itself to easy degradation and adverselycontributes to the landfill issue.

Cellulose is the most abundant organic polymer on earth, having anestimated annual production of 7.5×10¹⁰ tons. Nanocrystalline cellulose(NCC) is usually obtained by acid hydrolysis of cellulose, mainly fromplants, such as wood, cotton, and some other natural products such asbacteria and tunicate. NCC exhibits a highly crystalline characteristicand possesses several promising advantages such as nanoscale dimensions,hydroxy reactivity, large surface to volume ratio, relatively lowthermal conductivity, high mechanical strength, biocompatibility, andbiodegradability. These interesting physicochemical properties push NCCtowards a wide range of potential applications and have attractedtremendous attentions from both academia and industry. NCC have beenstudied and applied as an alternative material for biomedicalapplications because of its biocompatibility and biodegradability. Ithas also been examined as a reinforcing additive in severalnanocomposites for the fabrication of nanofibers, films, and foams.

Recently, cellulose-based lightweight foams have been the focus ofseveral studies. A cellulose nanofiber (CNF)-based foam with ultralowdensity and excellent thermal insulating capability using a freezecasting process has been reported. Boric acid was used as a crosslinkingagent, and graphene oxide was introduced to enhance the mechanicalperformance of the foams. The reported foams are highly anisotropic andstill weaker than the commercially viable petroleum-based counterparts;they show insignificant load resistance in the radial direction (˜2 kPaat 50% stain) and their strength reaches ˜22 kPa at 50% strain in theaxial direction with minimal elasticity behavior up to only ˜5% strain.In addition, they also exhibit a relatively high thermal conductivity inthe axial direction (>0.160 Wm⁻¹K⁻¹). Moreover, graphene oxide mayadversely contribute to the overall thermal insulation behavior, due toits high thermal conductivity. They also showed that the mechanicalproperties of cellulose-based foams decrease upon exposure to highertemperature and relative humidity. When the exposure condition changesform 23° C./50% RH to 30° C./85% RH, the retained modulus of the foam isabout 60% of the initial modulus and it is recovered to ˜80% upon dryingto 23° C./50% RH condition.

Thus, a need exists for renewable, greener, and cleaner alternativeinsulation materials that are mechanically robust and exhibit excellentthermal insulation capability achieving thermal conductivity (λ) valuesbelow or similar to those of the currently used commercial insulationmaterials, such as expanded or extruded polystyrene (λ=0.030-0.044Wm⁻¹K⁻¹), mineral wood (λ=0.030-0.040 Wm⁻¹K⁻¹), and cork (λ=0.040-0.050Wm⁻¹K⁻¹).

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, the disclosure provides a composite foam, comprising:

about 10 wt % to about 95 wt % of a cellulose component comprising about20% to about 100% cellulose nanomaterial;

about 1 wt % to about 25 wt % of a polymer comprising a plurality offirst crosslinkable groups,

about 1 wt % to about 25 wt % of a crosslinking agent comprising aplurality of second crosslinkable groups;

wherein at least a portion of the second crosslinkable groups formscovalent bonds with at least a portion of the first crosslinkable groupsand at least a portion of the hydroxy groups of the cellulose component.

In some embodiments, the composite foam has a porosity of about 30% toabout 99%. In some embodiments, wherein the composite foam has anapparent density of about 0.01 g/cm³ to about 1 g/cm³. In someembodiments, the cellulose component comprises 100% cellulosenanomaterial.

In some embodiments, the polymer is water-soluble. In some embodiments,the first crosslinkable group is a hydroxy, a thiol, a hydrazide, anamino group, or a combination thereof. In some embodiments, the firstcrosslinkable group is a hydroxyl (OH). In some embodiments, the secondcrosslinkable group is a carboxylic acid, an anhydride, an activatedester, an aldehyde, a ketone, or a combination thereof. In someembodiments, the second crosslinkable group is a carboxylic acid.

In some embodiments, the crosslinking agent is water-soluble. In someembodiments, the water-soluble polymer is polyvinyl alcohol (PVA),polyethylene glycol (PEG), polyacrylic acid (PAA), polyvinyl pyrrolidone(PVP), polyacrylamide (PAM), alginic acid, starch, xanthan gum, dextran,pectin, or a combination thereof. In some embodiments, the water-solublecrosslinking agent is 1,2,3,4-butane tetracarboxylic acid, oxalic acid,succinic acid, malic acid, citric acid, adipic acid, or a combinationthereof.

In some embodiments, the composite foam is formed by freeze-casting of aprecursor composition that does not comprise an organic solvent.

In some embodiments, the composite foam has a compression stressmeasured at 50% strain from about 50 kPa to about 250 kPa. In someembodiments, the composite foam has thermal conductivity from about0.015 Wm⁻¹K⁻¹ to about 0.045 Wm⁻¹K⁻¹. In some embodiments, the compositefoam has an elastic strain of at least 10% at Young's modulus of from 50kPa to about 1500 kPa.

In some embodiments, the composite foam comprises a colorant. In someembodiments, at least a portion of the cellulose component is silanized.In some embodiments, at least a portion of the hydroxy groups of thecellulose component are silanized.

In another aspect, provided herein is an article of manufacture, such asa building product or a consumer product comprising the composite foamof the disclosure. In some embodiments, the building product is aninsulation panel. In some embodiments, the consumer product is a cup, aplate, a shipping container, a cooler, or a potting container.

In another aspect, provided herein is a method of making a compositefoam, comprising freeze-casting or freeze-drying an aqueous compositioncomprising a foam precursor, wherein the foam precursor comprises:

about 10 wt % to about 95 wt % of a cellulose component comprising about20% to about 100% cellulose nanomaterial;

about 1 wt % to about 25 wt % of a polymer (e.g., a water-solublepolymer) comprising a plurality of first crosslinkable groups,

about 1 wt % to about 25 wt % of a crosslinking agent (e.g., awater-soluble crosslinking agent) comprising a plurality of secondcrosslinkable groups;

wherein at least a portion of the second crosslinkable groups can formcovalent bonds with at least a portion of the first crosslinkable groupsand at least a portion of the hydroxy groups of the cellulose component.

In some embodiments, the aqueous composition, such as suspension orsolution, does not include an organic solvent.

In another aspect, the disclosure provides composite foams prepared bythe method of the disclosure.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates preparation of an exemplary foam.

FIG. 2 shows a sample of an exemplary foam prepared from nanofiberderived from sawdust. The total weight of foam shown is 0.0412 gram witha density 0.02 g cm⁻³.

FIG. 3 is a schematic illustration of the ester bonds and hydrogen bondsbetween the components (NCC, PVA, and BTCA) in an exemplary foam.

FIG. 4 is a schematic illustration of the chemical bond formationbetween cellulose fiber and an exemplary crosslinking agent (citricacid).

FIG. 5 shows IR spectra of nanocrystalline cellulose (NCC) and NCC-basedcomposite foams: exemplary foam 10PVA/25BTCA/NCC (a), BTCA (b),10PVA/NCC (c) and NCC (d).

FIGS. 6A and 6B are stress-strain curves of NCC composite foams withvarious contents of PVA and BTCA.

FIGS. 7A and 7B show stress at 50% strain values (7A), and modulus ofelasticity (7B) of NCC composite foams with various contents of PVA andBTCA. Error bars show ±standard deviations.

FIG. 8 shows thermal conductivity values of NCC composite foams withvarious contents of PVA and BTCA. Error bars show ±standard deviations.

FIG. 9 depicts vertical burning test (UL94) of an exemplary foam(10P/25BTCA/NCC).

FIGS. 10A and 10B are photographs of an exemplary foam (10P/25BTCA/NCC)before and after carbonization under N₂ (10A) and SEM micrograph aftercarbonization (10B).

FIGS. 11A-11F are photographs of various composite foams: NCC (11A),NCC/BTCA (11B), NCC/PVA (11C), NCC/PVA/BTCA (11D), NCC/PVA/CA (11E), andwood fiber/PVA/CA (11F); all photos have the same scale bar as shown inFIG. 11D.

FIGS. 12A-12F are SEM images of NCC-based composite foam materials:SPVA-NCC (12A), 10PVA-NCC (12B), 20PVA-NCC (12C), 10PVA/10BTCA/NCC(12D), 10PVA/25BTCA/NCC (12E), and 20PVA/10BTCA/NCC (12F).

FIG. 13 is an illustration of foam preparation process involving ahydrophobization step.

FIG. 14 is a thermal conductivity comparison of commercial insulationmaterials with an exemplary of 10PVA/25BTCA/NCC foam.

FIGS. 15A and 15B demonstrate that an exemplary foam exhibits lightweight and good mechanical properties.

FIG. 16 demonstrates that exemplary foams can be prepared with differentdensities; from left to right: 0.01 g/cm³ (NCC foam), 0.027 g/cm³ (2NCCfoam), 0.045 g/cm³ (4NCC foam), 0.1 g/cm³ (8NCC foam), with PVA, citricacid, and catalyst (KH₂PO₄).

FIG. 17 demonstrates that the foams can be molded to any shape with goodmechanical strength.

FIGS. 18A-18D are low magnification (18A) and high magnification (18B)SEM micrographs of pure NCC foam and low magnification (18C) and highmagnification (18D) SEM micrographs of 2NCC (10PVA/25BTCA/NCC) compositefoam.

FIGS. 19A and 19B are SEM images of NCC foam pre-frozen at −20° C. (19A)and in liquid nitrogen (19B).

FIGS. 20A and 20B are SEM images of a silane agent-modified exemplaryhydrophobic NCC foam (density of 0.027 g/cm³).

FIG. 20C is a graph of a compression test comparison of an exemplaryhydrophobic (MTMS-treated) foam with Styrofoam.

FIG. 21 shows stress-strain curves of Styrofoam and exemplary NCCcomposite foams with various densities (0.01 g/cm³ (NCC foam), 0.027g/cm³ (2NCC foam), 0.045 g/cm³ (4NCC foam), 0.1 g/cm³ (8NCC foam)).

FIGS. 22A-22D are a photograph (22A) and SEM images (22B-D) of anexemplary foam 2NCC (NCC, PAA and PVA; wt % PVA:PAA=1:1).

FIGS. 23A and 23B are SEM images of an exemplary foam (2CNF/PVA/BTCAfoam).

DETAILED DESCRIPTION

Cellulose nanomaterials are unique materials with high mechanicalstrength and a number of other attractive physical properties. However,most of these physical properties have only been demonstrated at thenano-whiskers level. It has been difficult to realize these propertiesin bulk materials. The inventors found that incorporating certain typesof crosslinking agents into cellulose nanomaterials under certainconditions surprisingly leads to formation of chemical bonds that inturn create well-organized cellular structures that afford the foam withhigh mechanical properties while maintaining a high porosity.

Thus, in one aspect, provided herein is a composite foam, comprising acomposition formed by combining a cellulose component comprising about20% to about 100% cellulose nanomaterial, a water-soluble polymercomprising a plurality of first crosslinkable groups, a water-solublecrosslinking agent comprising a plurality of second crosslinkablegroups, wherein at least a portion of the second crosslinkable groupsforms covalent bonds with at least a portion of the first crosslinkablegroups and at least a portion of the hydroxy groups of the cellulosecomponent. In some embodiments, the components of the composite foam arecovalently liked.

The composite foams of the disclosure comprise a cellulose component. Insome embodiments, the composite foams comprise about 10 wt % to about 95wt % of the cellulose component. In some embodiments, the compositefoams comprise about 20 wt % to about 95 wt % of the cellulosecomponent. In some embodiments, the composite foams comprise about 30 wt% to about 95 wt % of the cellulose component. In some embodiments, thecomposite foams comprise about 40 wt % to about 95 wt % of the cellulosecomponent. In some embodiments, the composite foams comprise about 60 wt% to about 95 wt % of the cellulose component. In some embodiments, thecomposite foams comprise about 60 wt % to about 85 wt % of the cellulosecomponent. In some embodiments, the composite foams comprise about 70 wt% to about 85 wt % of the cellulose component. In some embodiments, thecomposite foams comprise about 60 wt % to about 80 wt % of the cellulosecomponent.

In some embodiments, the cellulose component comprises about 20% toabout 100%, about 30% to about 100%, about 40% to about 100%, about 50%to about 100%, about 60% to about 100%, about 70% to about 100%, about80% to about 100%, or about 90% to about 100% of a cellulosenanomaterial. In some embodiments, the cellulose component comprises100% cellulose nanomaterial. In some embodiments, the cellulose consistsessentially of a cellulose nanomaterial.

As used herein, “cellulose nanomaterials” or “nanocellulose” refer tonano-structured cellulose, including but not limited to nanocrystallinecellulose or cellulose nanocrystals (denoted herein as CNC or NCC, usedinterchangeably), cellulose nanofibers (CNF), nanofibrillated cellulose(NFC), and nano-structured cellulose produced by bacteria (bacterialnanocellulose). In some embodiments, nanocellulose can be obtained fromnative cellulose fibers by an acid hydrolysis. In some embodiments, thecellulose nanomaterial is nanocrystalline cellulose (NCC). In additionto cellulose nanomaterials, in some embodiments, other plant-basedfibrous materials (e.g. wood-derived nanofibrils) can be included in thecellulose component. In some embodiments, in addition to the cellulosenanomaterial, the cellulose component comprises cellulose that is notnanoscale cellulose. In some embodiments, the cellulose component caninclude wood pulp. In some embodiments, the cellulose component cancomprise cellulose, lignin, hemicellulose, and combinations thereof. Insome embodiments, the cellulose component can comprise a carbohydrate,including but not limited to cellulose, starch, xylan, fructan, pectin,hemicellulose, and combinations thereof.

The composite foams of the disclosure further comprise about 1 wt % toabout 25 wt % of a water-soluble polymer comprising a plurality of firstcrosslinkable groups. In some embodiments, the composite foams compriseabout 5 wt % to about 25 wt % of a water-soluble polymer. In someembodiments, the composite foams comprise about 5 wt % to about 20 wt %of a water-soluble polymer. In some embodiments, the composite foamscomprise about 10 wt % to about 20 wt % of a water-soluble polymer. Anysuitable water-soluble polymer can be used in the compositions disclosedherein. Examples of suitable water-soluble polymers include but are notlimited to polyvinyl alcohol (PVA), polyethylene glycol (PEG),polyacrylic acid (PAA), polyvinyl pyrrolidone (PVP), polyacrylamide(PAM), alginic acid, starch, xanthan gum, dextran, pectin, or acombination thereof. In some embodiments, the water-soluble polymer ispolyvinyl alcohol (PVA).

The water-soluble polymers comprise a plurality of first crosslinkablegroups. In some embodiments, the first crosslinkable group is a hydroxy,a thiol, an aldehyde, a ketone, a hydrazide, an amino group, or acombination thereof.

The composite foams of the disclosure further comprise about 1 wt % toabout 25 wt % of a water-soluble crosslinking agent comprising aplurality of second crosslinkable groups, wherein the secondcrosslinkable group is a group capable of forming a covalent bond withthe hydroxyl groups (OH) of the cellulose component and with the firstcrosslinkable group. In some embodiments, the composite foams compriseabout 5 wt % to about 25 wt % of a water-soluble crosslinking agent. Insome embodiments, the composite foams comprise about 10 wt % to about 25wt % of a water-soluble crosslinking agent. In some embodiments, thecomposite foams comprise about 10 wt % to about 20 wt % of awater-soluble crosslinking agent.

In some embodiments, the second crosslinkable group is a carboxylicacid, an anhydride, an activated ester, an aldehyde, a ketone, or acombination thereof. In some embodiments, the second crosslinkable groupis a carboxylic acid. In some embodiments, the water-solublecrosslinking agent comprises two or more carboxylic acid groups oractivated esters thereof.

Any suitable water-soluble crosslinking agent can be used in thecomposite foams of the disclosure. In some embodiments, thewater-soluble crosslinking agent is a polycarboxylic acid. In someembodiments, the water-soluble crosslinking agent is 1,2,3,4-butanetetracarboxylic acid (BTCA), oxalic acid, succinic acid, malic acid,citric acid, adipic acid, or a combination thereof. In some embodiments,the water-soluble crosslinking agent is 1,2,3,4-butane tetracarboxylicacid (BTCA). In some embodiments, the water-soluble crosslinking agentis citric acid.

Without wishing to be bound by theory, the formation of covalent bondsbetween an exemplary crosslinking agent and NCC is shown in FIG. 3. Asan example, a crosslinking agent, BTCA, contains four carboxyl acidgroups and can form ester bond with hydroxyl group (OH). The cellulosecomponent constituents (NCC, cellulose, hemicellulose, lignin, andothers) all contain hydroxyl groups and can thus form chemical bondswith such crosslinking agents. Co-polymer(s) possessing hydroxyl group,such as PVA, can also react with such crosslinking agents. Thecrosslinking agents can form chemical bonds with multiple constituentsof the composite foams of the disclosure (e.g., cellulose,hemicellulose, lignin, PVA, etc.). The addition of catalysts (such asK₂HPO₄) or activating agents (such as N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide) can be also used to expedite the formation of covalentbonds between the crosslinking agent and the polymer and between thecrosslinking agent and the cellulose component. The formation ofcovalent bonds between the components can provide the structureintegrity of the composite foams. In some embodiments, crosslinkingcreates a well-organized cellular structure that affords the compositefoam with high porosity and high strength.

The composite foams of the disclosure are highly porous. In someembodiments, the composite foam has a porosity of about 30% to about99%. In some embodiments, the composite foam has a porosity of about 40%to about 99%. In some embodiments, the composite foam has a porosity ofabout 50% to about 99%. In some embodiments, the composite foam has aporosity of about 60% to about 99%. In some embodiments, the compositefoam has a porosity of about 70% to about 99%. In some embodiments, thecomposite foam has a porosity of about 80% to about 99%. In someembodiments, the composite foam has a porosity of about 90% to about99%. In some embodiments, the composite foams have a well-organizedstructure with pores that are inter-connected through solid andconsistent cell walls. In some embodiments, the composite foams havepores with an average pore size of about 50 μm to about 100 μm asdetermined by SEM micrographs,

The composite foams of the disclosure are lightweight. In someembodiments, the composite foam has a density of about 0.01 g/cm³ toabout 1 g/cm³. In some embodiments, the composite foam has a density ofabout 0.01 g/cm³ to about 0.5 g/cm³. In some embodiments, the compositefoam has a density of about 0.01 g/cm³ to about 0.25 g/cm³. In someembodiments, the composite foam has a density of about 0.01 g/cm³ toabout 0.08 g/cm³. In some embodiments, the composite foam has a densitycomparable or equal to the density of Styrofoam.

The composite foams of the disclosure exhibit high mechanicalproperties. In some embodiments, the composite foam has a compressionstress measured at 50% strain from about 5 kPa to about 400 kPa. In someembodiments, the composite foam has a compression stress measured at 50%strain from about 50 kPa to about 300 kPa. In some embodiments, thecomposite foam has a compression stress measured at 50% strain fromabout 50 kPa to about 250 kPa. In some embodiments, the composite foamhas a compression stress measured at 50% strain from about 100 kPa toabout 400 kPa. In some embodiments, the composite foam has an elasticstrain of at least 10% at Young's modulus of from 50 kPa to about 1500kPa. In some embodiments, the composite foam has an elastic strain of atleast 10% at Young's modulus of from 100 kPa to about 1300 kPa. In someembodiments, the composite foam has an elastic strain of at least 10% atYoung's modulus of from 500 kPa to about 1300 kPa. In some embodiments,the composite foams of the disclosure have mechanical propertiescomparable to or equal to those of Styrofoam.

The composite foams of the disclosure exhibit low thermal conductivityand thus can be used for thermal insulation. In some embodiments, thecomposite foam has thermal conductivity from about 0.015 Wm⁻¹K⁻¹ toabout 0.045 Wm⁻¹K⁻¹. In some embodiments, the composite foam has thermalconductivity from about 0.020 Wm⁻¹K⁻¹ to about 0.045 Wm⁻¹K⁻¹. In someembodiments, the composite foam has thermal conductivity from about0.020 Wm⁻¹K⁻¹ to about 0.035 Wm⁻¹K⁻¹. In some embodiments, the compositefoams have a thermal conductivity below the range for commoncommercially available insulation materials, such as expandedpolystyrene (λ=0.030-0.040 Wm⁻¹K⁻¹), mineral wool (λ=0.030-0.040Wm⁻¹K⁻¹, and extruded polystyrene, λ=0.033-0.044 Wm⁻¹K⁻¹).

In some embodiments, colorants, such as inorganic compounds or organicdyes, can be added to the composite foams. Suitable colorants includemetal oxides and any dye that can be used as a colorant for cellulose,including but not limited to fiber-reactive dyes, direct dyes, and vatdyes. In some embodiments, the colorant is a fiber-reactive dye that canform covalent bonds with cellulose, for example, dichlorotriazine,monochlorotriazine, aminochlorotriazine dye. Direct dyes include watersoluble salts of azo and polyazo compounds. Vat dues include indigo dyesand synthetically made equivalents.

In some embodiments, the composite foams of the disclosure arehydrophobic. Hydrophobization of the foams of the disclosure can beachieved in any suitable manner, for example, treating the foam or itsprecursor cellulose component with a silanizing agent (e.g., analkoxysilane such as MTMS), as shown in FIG. 13. Thus, in someembodiments, at least a portion of the hydroxy groups of the cellulosecomponent are silanized.

In another aspect, the disclosure provides a method of making acomposite foam, comprising freeze-casting (or freeze-drying) an aqueouscomposition comprising about 0.01 wt % to about 40 wt. % of a foamprecursor mixture. In some embodiments, the aqueous compositioncomprises about 0.01 wt % to about 30 wt. % of a foam precursor mixture.In some embodiments, the aqueous composition comprises about 0.01 wt %to about 25 wt. % of a foam precursor mixture. In some embodiments, theaqueous composition comprises about 0.01 wt % to about 20 wt. % of afoam precursor mixture. In some embodiments, the aqueous compositioncomprises about 0.01 wt % to about 15 wt. % of a foam precursor mixture.In some embodiments, the aqueous composition comprises about 0.01 wt %to about 10 wt. % of a foam precursor mixture. In some embodiments, theaqueous composition comprises about 0.01 wt % to about 5 wt. % of a foamprecursor mixture. In some embodiments, the aqueous compositioncomprises about 0.1 wt % to about 30 wt. % of a foam precursor mixture.

In some embodiments, the foam precursor mixture comprises: about 40 wt %to about 95 wt % of a cellulose component comprising about 20% to about100% cellulose nanomaterial; about 1 wt % to about 25 wt % of awater-soluble polymer comprising a plurality of first crosslinkablegroups, about 1 wt % to about 25 wt % of a water-soluble crosslinkingagent comprising a plurality of second crosslinkable groups; wherein atleast a portion of the second crosslinkable groups can form covalentbonds with at least a portion of the first crosslinkable groups and atleast a portion of the hydroxy groups of the cellulose component. Insome embodiments, the aqueous composition is a suspension. In someembodiments, the aqueous composition does not include an organicsolvent. In some embodiments, the composite foams of the disclosure areformed by freeze-casting or freeze-drying a precursor composition thatdoes not comprise an organic solvent.

General processes for the preparation of the composite foams of thedisclosure are illustrated in FIGS. 1 and 13. In some embodiments, to anaqueous composition comprising suitable amounts of a cellulose precursorand a polymer comprising a plurality of first crosslinkable groups, oneor more crosslinking agents, such as those described above, can be addedand the resulting mixture can be maintained at a suitable temperaturefor a time sufficient to form chemical linkages between the firstcrosslinkable groups and second crosslinkable groups and between thehydroxyl groups of the cellulose component and the second crosslinkablegroups. Following the crosslinking reaction, freeze casting orfreeze-drying can be applied to form the composite foam. Apost-treatment can be applied to enhance the degree of crosslinking andfurther improve the properties of the final foam products. In someembodiments, the post treatment is a hydrophobization process such assilanization described above. The methods disclosed herein yieldcomposite foams with high porosity and desirable mechanical properties,such as those described above.

The foams of the disclosure have a wide variety of applications. In anaspect, the disclosure provides an article of manufacture comprising acomposite foam disclosed herein. In some embodiments, the article ofmanufacture is a construction material, such as an insulating panel. Insome embodiments, the insulation panel comprises composite foam of thedisclosure sandwiched between panels made from other materials. In someembodiments, the article of manufacture is a consumer product, such as acup, a plate, a bowl, a shipping container, a cooler, or a pottingcontainer. In some embodiments, the article of manufacture iswater-resistant or waterproof. Any product made of Styrofoam can bemanufactured from the composite foams of the disclosure.

As used herein, the term “about” includes ±10% of the stated value.

While each of the elements of the present disclosure is described hereinas containing multiple embodiments, it should be understood that, unlessindicated otherwise, each of the embodiments of a given element of thepresent invention is capable of being used with each of the embodimentsof the other elements of the present disclosure and each such use isintended to form a distinct embodiment of the present disclosure.

The referenced patents, patent applications, and scientific literaturereferred to herein are hereby incorporated by reference in theirentirety as if each individual publication, patent or patent applicationwere specifically and individually indicated to be incorporated byreference. Any conflict between any reference cited herein and thespecific teachings of this specification shall be resolved in favor ofthe latter. Likewise, any conflict between an art-understood definitionof a word or phrase and a definition of the word or phrase asspecifically taught in this specification shall be resolved in favor ofthe latter.

As can be appreciated from the disclosure above, the present inventionhas a wide variety of applications. The invention is further illustratedby the following examples, which are only illustrative and are notintended to limit the definition and scope of the invention in any way.

EXAMPLES

A green method to prepare an environmentally friendly thermal insulationfoam based on NCC is provided. Water is the only solvent used in thismethod without any excessive temperature or pressure. Polyvinyl alcohol(PVA), a water-soluble polymer with no toxic property is used in theformulation to increase the elastic properties of the composite foams.1,2,3,4-butane tetracarboxylic acid (BTCA) is also introduced as acrosslinking agent to chemically bond NCC and PVA to obtain highstrength. For the first time, provided are NCC-PVA foams that exhibitsuperior properties to those of previously reported nanocellulose-basedfoams in terms of the combination of required properties, i.e., thermalinsulation, elasticity, and strength.

1.1. Materials

NCC was extracted from fully bleached hardwood kraft pulp by sulfuricacid treatment, following a previously established method (Dong, Revol,& Gray, 1998; Hamad & Hu, 2010). In brief, the pulp was first groundedby a Wiley mill and then passed through two mesh screens of 40 and 60.Those retained by the 60 mesh screen were collected and hydrolyzed using64.5% w/w sulfuric acid at 45° C. for 25 min. Excessive deionized waterwas then added to quench hydrolysis reaction. The ensuingnanocrystalline cellulose suspension was transferred to a dialysistubing and dialyzed against running distilled water for at least fivedays to remove the acid and soluble sugar. All other materials were usedas received. Polyvinyl alcohol (PVA) with a molecular weight ofM_(w)=85000-124000 and 99% hydrolyzed was purchased from Sigma Aldrich.The crosslinking agent used was 1,2,3,4-butane tetracarboxylic acid(BTCA), supplied by Acros Organics and dipotassium phosphate (DP) waspurchased from Sigma Aldrich and used as a catalyst. Distilled water wasused in all the experiments without further purification.

1.2. Preparation of NCC-Based Foams

NCC powder was dispersed in deionized water (2 g NCC per 98 ml of water)under sonication for 1 h. Polyvinyl alcohol (PVA) was then added to theNCC solution and heated at 90° C. for 2 h for PVA complete dissolution.The weight ratios tested for PVA:NCC were 0.00:1.00, 0.05:1.00,0.10:1.00 and 0.20:1.00, described as pure NCC, SPVA/NCC, 10PVA/NCC, and20PVA/NCC, respectively. BTCA and DP were also added to 10PVA/NCC and20PVA/NCC solutions to act as a crosslinking agent and catalyst,respectively (with a 2:1 w/w ratio of BTCA/DP) at two dosages of 10% and25% of NCC weight. The final products were described as10PVA/10BTCA/NCC, 10PVA/25BTCA/NCC, and 20PVA/10BTCA/NCC.10PVA/25BTCA/NCC, for instance, was composed of 74 wt. % NCC, 7.5 wt. %PVA, and 18.5 wt. % BTCA/DP with respect to the total weight of thefinal composite. The mixture solutions were then placed in apolytetrafluoroethylene tube and kept in a freezer (−20° C.) for 6 h forcomplete water solidification. The frozen samples were transferred tothe freeze dryer and freeze dried for 48 h to sublimize the water andobtain the foam. VirTis Lyo-Centre 3.5 L DBT ES-55 Benchtop Freeze Dryerwas used at a controlled temperature of −50° C.

1.3. Fire Resistance and Carbonization

Vertical burning tests were done in an open-air environment followingUL94. To further investigate the thermal stability and also assess thepotential of these foams as precursors for carbon foams, carbonizationwas also conducted. The 10PVA/25BTCA/NCC foam was carbonized under atwo-stage heating program with the protection of nitrogen. First, thesample temperature was raised to 500° C. at a heating rate of 2° C./minand it was kept for 2 h at the stabilized temperature. Then, it washeated up at a rate of 1° C./min to reach 800° C. and further carbonizedat that temperature for 3 h. Samples were then taken out after coolingto room temperature inside the tube.

2. Characterization

Scanning electron microscopy (SEM) images were taken on an FEI SEMQuanta 200 F to investigate the cellular morphology and porosity of thefoams. The samples were sputtered by gold and imaged at 10 kV. Fouriertransform infrared spectroscopy (FTIR) was conducted on a NEXUS 670spectrometer from 4000-400 cm⁻¹. Thermogravimetric analysis (TGA) wascarried out on an SDT Q600 TG thermal analyser under air condition,heating from 25 to 800° C. at a heating rate of 10° C./min. The atomicforce microscopy (AFM) was conducted on a Bruker MultiMode 8 AFMinstrument, equipped with Silicon SPM-sensor. The data was obtained onnon-contact/tapping mode, ScanAsyst® peak force tapping AFM probes, withresonance frequency of 190 kHz, and a force constant of 48 N/m. Verydilute NCC water solution was ultrasonicated and sprayed on a siliconwafer and dried before the AFM test. Thermal conductivity measurementswere done using a ThermTest HotDisk TP-500 transient thermal analyzer. A6 mm disk-shaped Kapton-wrapped sensor was sandwiched between the twohalves of the samples. Certain electrical power and time were applied toobtain accurate thermal conductivity measurements. 6 to 10 replicationswere conducted for each sample and their average values and standarddeviations are reported.

To determine the mechanical properties of NCC composite foams, a customsmall-scale benchtop compression test apparatus was used. Thedisplacement rate for all the tests was set at 0.01 mm/s. The topportion of the compression fixture had a moving frictionless flatsurface and the bottom portion of the fixture was a stationary flatplate. Test samples were cylindrical with a height (38.1 mm) that was1.5 times of the diameter (25.4 mm) to avoid buckling during thecompression. At least five samples were tested for each condition andthe average and standard deviation values are reported.

3. Results and Discussion

The NCC powder was derived from the acid hydrolysis of natural biomassmaterial, and the NCC aqueous solution exhibited a transparentappearance with a light blue color, indicating the good dispersion ofthe NCC in water. A highly lightweight, relatively strong, andsuper-insulative exemplary foam was obtained after adding PVA and BTCAcross-linking agent to NCC. The porosity, stress at 50% strain, and thethermal conductivity were measured to be 98.22%, 73 kPa, and 0.027Wm⁻¹K⁻¹, respectively. Scanning electron microscopy showed a highlymacro-porous nature of the composite foam with a relatively isotropicstructure. Such a highly porous structure explains the lightweight andgood thermal insulation performance of the foams. The exemplary foam(10PVA/25BTCA/NCC foam) showed excellent mechanical strength and couldwithstand a load of 100 g without any shape distortion for an extendedperiod of time. The load was 200 times of the sample's weight. (FIG. 2)Pure NCC foam crashed immediately under this load. Fire resistance testsalso revealed that the 10PVA/25BTCA/NCC foam did not produce any ash; itcould maintain its overall structure with some volumetric shrinkage.

3.1. Morphology of Cellulose Nanocrystals and Resultant Foams

As determined by AFM images, NCC nanocrystals exhibited relativelyuniform size and distribution, having around 20-40 nm width and 100-200nm length, with a surface height ranging from 3-7 nm.

The NCC-based composite foams were in monolithic condition and exhibitedmacro-porous structure. To give a view of the structure and porosity,SEM was conducted. As the low magnification SEM micrograph of FIG. 18Ashows, the pure NCC foam exhibited a sheet-like assembled structure witha high degree of disorder, where consistent and intact cell walls couldnot be developed (FIG. 18B). The other foams with different amount ofPVA and BTCA loading showed similar morphology as that of pure NCC.However, 10PVA/25BTCA/NCC foams exhibited a well-organized structurewith significantly smaller pores that are inter-connected through solidand consistent cell walls (FIGS. 18C and 18D). Based on thehigh-magnification SEM micrographs (FIG. 18D), an average pore size of50-100 μm was estimated for the exemplary 10PVA/25BTCA/NCC compositefoam. Compared to the pure NCC foam, the well-defined cell walls andpores with smaller sizes in the composite foam enhanced both the thermalinsulation and the mechanical performance.

3.2. Density and Porosity of Foams

The apparent density of the foams, ρ_(foam) is calculated based on Eq.(1) using the measured weight and volume of the foams. The mass wasmeasured using a high precision analytical balance and the apparentvolume was calculated by measuring the dimensions of the samples using acaliper. Average density values were calculated using at least fivesamples.

$\begin{matrix}{\rho_{foam} = \frac{m}{V}} & (1)\end{matrix}$

The theoretical density of the bulk un-foamed composite, ρ_(un-foamed)is obtained by the rule of mixture, Eq. (2), from the densities ofcomponents (ρ_(i)) and their weight fractions (W_(i)) using thefollowing density values: NCC=1.460 g·cm⁻³, PVA=1.190 g·cm⁻³, BTCA=1.674g·cm⁻³ and DP=2.440 g·cm⁻³ (Svagan et al., 2008; Wicklein et al., 2015).

$\begin{matrix}{\rho_{{un}\text{-}{foamed}} = \frac{1}{\Sigma_{i = 1}^{n}( {W_{i}\text{/}\rho_{i}} )}} & (2)\end{matrix}$

Porosity was then measured using Eq. (3) (Wang et al., 2014;).

$\begin{matrix}{{\text{Porosity~~}(\%)} = {( {1 - \frac{\rho_{foam}}{\rho_{{un}\text{-}{foamed}}}} ) \times 100{\%.}}} & (3)\end{matrix}$

The apparent density of the NCC-based composite foams were measured tobe in the range of 0.020-0.027 g·cm⁻³, and the corresponding porosityvalues ranged from 98.6% to 98.1%, as listed in Table 1. The apparentdensity of the resultant foam can be tuned with the solvent and soluteratio during the mixing step. For the current study, 2 wt. % NCC wasadded to 98 wt. % water, which resulted in a final density of 0.020g·cm⁻³ for pure NCC foam. As seen in Table 1, the density continuouslyincreased with an increase in the PVA and BTCA content. There is anincrease of 30% in density when PVA content was increased from 0 to 20%,which corresponds to ˜0.4% reduction in the porosity. The major reasonfor an increased density with the addition of PVA and BTCA is that theNCC mass in the solution was kept unchanged and as PVA and BTCA wereadded, the total mass was increased while the apparent volume was keptconstant. This resulted in slightly greater density values for thecomposite foams. Therefore, smaller density values for composite foamscan also be achieved by decreasing solute to solvent ratio duringmixing.

TABLE 1 Density and porosity values of NCC composite foams. ApparentDensity of density of unfoamed composite foam, composite foam, Porosity,Composition g cm⁻³ g cm⁻³ % NCC 0.020 1.46 98.63 5PVA/NCC 0.022 1.4598.48 10PVA/NCC 0.024 1.44 98.33 20PVA/NCC 0.026 1.42 98.1710PVA/10BTCA/NCC 0.026 1.47 98.23 10PVA/25BTCA/NCC 0.027 1.52 98.2220PVA/10BTCA/NCC 0.027 1.45 98.14

3.3. NCC, PVA and BTCA Interactions

The crosslinking procedure of NCC, PVA, and BTCA is shown in FIG. 3.BTCA contains four carboxyl acid groups and can esterify with varioushydroxy groups (R—OH), as it has been used for cellulose crosslinking.With the presence of DP catalyst, anhydride was formed in the solutionand facilitated the ester bond formation between the BTCA and thehydroxy groups of NCC. On the other hand, BTCA could also form esterbonds with the hydroxy groups of PVA. This means the BTCA could formchemical bonds with both NCC and PVA and acted as a strong chemicalbinder between them. The resultant foam behaved as an intact compositematerial wherein the characteristics of both NCC and PVA were reflectedin the final properties. At the same time, intramolecular andintermolecular hydrogen bonds formed between hydroxy groups within andamong PVA and NCC as shown in FIG. 3. The mechanical strength of thecomposite foams was thus further improved by the crosslinkedhydrogen-bonded network structure.

Infrared spectra were used to investigate the interactions between PVA,NCC and BTCA at different material compositions. As shown in FIG. 5, thewide peak around 3500 cm⁻¹ of 10PVA/25BTCA/NCC composite foamcorresponds to the hydroxyl bonds between PVA and NCC, and the adsorbedwater on their surfaces. The absorption peak at 1644 cm⁻¹ is alsoattributed to C—O(H) stretching vibrations. The characteristic IR bondlocated at 1706 cm⁻¹ is associated with carboxyl groups. Compared tocarboxyl group of BTCA, the composite foam showed a small sharp peakaround 1554 cm⁻¹, confirming that the ester bond was formed.

3.4. Thermal Stability

Thermogravimetric measurement in air condition was applied to evaluatethe thermal stability of the composite foams. Both pure NCC foam and10PVA/25BTCA/NCC foam experienced an initial weight loss of ˜6% whenheated up to 100° C.; this is attributed to the water adsorbed on thesurface. PVA had insignificant weight loss before 150° C. and started todecompose at ˜200° C. BTCA showed a good thermal stability up to 170°C., beyond which started to decompose rapidly. The significant mass lossof 10PVA/25BTCA/NCC foam, associated with the degradation anddecomposition was observed at ˜250° C.; this temperature was about 200°C. for BTCA, 230° C. for PVA and 280° C. for pure NCC foam. These valuesindicate that the thermal decomposition temperature of pure NCC slightlydecreases once BTCA and PVA are added. Since the decompositiontemperature of PVA and NCC are similar, this decrease is associated withthe lower decomposition temperature of BTCA. However, it is noted thatthis drop was less than 7% (˜20° C.).

3.5. Mechanical Properties

FIGS. 6A and 6B depict the compressive stress-strain curves for NCCcomposite foam samples with different formulations containing 0-20 wt. %PVA and 0-25 wt. % BTCA. Usually low-density foams exhibit three regionsof stress-statin curve under compression loading, namely, elasticregion, collapse region, and densification region. The pure NCC foamexhibited no elastic region. As the loading started, the collapse regionis immediately observed, indicating that there was no measurable elasticbehavior of pure NCC foam and the cell walls started to collapse, break,or buckle at the very early stages of the loading. Once the compactionreached to about 50% strain, the stress started to build up, which isdue to the densification of the collapsed cell walls. This stress-strainbehavior indicates that the pure NCC foam was very weak and evenmacroscopically collapsed at very delicate mechanical loads.

In PVA/NCC foams, once the PVA was introduced to the NCC structure, thestress-strain behavior was altered. Introducing 5 wt. % PVA did notcause a significant change in the overall behavior of NCC foam. However,the addition of 10 and 20 wt. % PVA to NCC created some load bearingcapability, which was greater at 20 wt. % PVA. It is also interesting tonote that the collapse region, i.e., the plateau region afterelasticity, was minimized in the cases of 10PVA/NCC and 20PVA/NCC. Thecompaction region is almost connected to the elastic region, whichindicates that the cell walls experienced more elastic deformationwithout significant buckling failure, before they become in contact withone another, where the compaction started, and the stress increasedsignificantly. However, the strength values were still low (<10 kPa).The relatively low strength values indicate that the cellulosenanocrystals with high strength could not effectively participate inload bearing; this is attributed to the lack of strong bonding betweenelastic PVA phase and rigid NCC phase. To further enhance the strengthof the foams, BTCA was introduced as the crosslinking agent between NCCand PVA. The addition of 10 wt. % BTCA to 10PVA/NCC and 20PVA/NCC didnot cause a significant change in the mechanical behavior, especially atthe early stages of the loading. It is believed that a sufficient degreeof crosslinking could not form in these cases. However, when 25 wt. %BTCA was added to 10PVA/NCC formulation, the mechanical strength of thefoams was significantly increased, reaching to a stress value of ˜40 kPain the elastic range at ˜13% strains and a stress value of ˜73 kPa at50% strain. As also seen in FIG. 6B, the 10PVA/25BTCA/NCC composite foamexhibited a dramatically increased stiffness at the first stage ofloading until 13% strain, under a linear stress-strain relationship.Once the loading entered the collapse region, the stress continued torise as strain was increased, indicating that the failure of the cellwalls under buckling occurred in a more gradual and ductile manner, asopposed to the sudden and brittle failure of the pure NCC foam. Theelastic range, strength, and stiffness values achieved here are superiorto those of previously reported nanocellulose-based foams.

As a measure of strength, FIG. 7A shows the stress at 50% strain for NCCcomposite foams. The pure NCC foam's stress was ˜4 kPa, whichcontinuously increased with an increase in the PVA content, reaching to28 kPa at 20 wt. % PVA. In the pure NCC foam, there were no strong bondsbetween the individual nanocrystals as they have emerged from acompletely dispersed cellulose solution, upon water solidification andremoval. However, in the case of PVA/NCC composite foams, once PVA iscompletely dispersed together with the cellulose nanocrystals, upon theice crystallization, the PVA molecules and the cellulose nanocrystalsare simultaneously ejected from the crystallized H₂O regions and locallyprecipitated with a uniform blend. As their local concentrationsincreases as a consequence of further ice crystallization, the PVAmolecules form entangled chain structures, while they encapsulate theindividual nanocrystals. This causes an effective physical bindingaction of PVA, where all nanocrystals are held together and thus thecomposite becomes stronger.

For 10PVA/25BTCA/NCC foam, the stress at 50% strain reached ˜73 kPa,accounting for ˜1820% increase in the strength of pure NCC foam. Thishuge enhancement is explained by the obtained 3-D chemically crosslinkednetworks of PVA and NCC with the aid of BTCA, as discussed earlier. Insuch a network, PVA provided the elasticity and ductility, while NCCcontributed to the enhanced strength and stiffness. It is also notedthat the macroscopic improvement in the cellular morphology is aconsequence of stronger material during cell formation, which in turn,contributes to the enhanced mechanical performance.

FIG. 7B shows the modulus of elasticity of the NCC composite foams. Themodulus was calculated by fitting a line for a segment of stress-straincurves where the strain is less than 10%. As discussed earlier, pure NCCfoams did not show any significant elasticity and therefore the moduluswas barely registered for a strain of 10% at ˜2 kPa. However, with theinclusion of PVA and BTCA to the foam structure, the modulus at 10%strain increased with an increase in the PVA content and reached to ˜250kPa for 10PVA/25BTCA/NCC, which is more than two orders of magnitudeenhancement. Table 2 provides a comparison of the strength (stress at50% strain) and modulus (at 10% strain) of the current foam with thedata of the published works based on nanocellulose. As seen in theTable, our composite foam showed the best combination of the higheststrength and the highest modulus. Both the strength and the modulus ofthe current foam are the highest when compared to the cellulose-basedfoams without any inorganic reinforcement. The performance is similar orbetter even when compared to the foams with reinforcing inorganicadditives.

TABLE 2 Comparison of stress at 50% strain and modulus with publishedworks. Stress Modulus at 50% at 10% strain strain Work Title (kPa) (kPa)Reference Thermally insulating and ~22* ~200* Nature fire-retardantlightweight Nanotechnology, anisotropic foams based 2015, 10, onnanocellulose and 277-283 graphene oxide Cellulose Nanocrystal  ~5* ~10* Advanced Materials, Aerogels as Universal 2015, 27, 3D LightweightSubstrates 6104-6109 for Supercapacitor Materials Superior mechanical~75  ~200* Journal of performance of highly the Mechanical porous,anisotropic Behavior of nanocellulose-montmorillonite Biomedicalaerogels prepared by Materials, freeze casting 2014, 37, 88-89 Highlyflexible ~4 ~10 Carbohydrate cross-linked cellulose Polymers nanofibrilsponge-like 179 (2018) 333-340 aerogels with improved mechanicalproperty and enhanced flame retardancy Composite foams of ~72  ~250  thedisclosure

3.6. Thermal Insulation

Thermal conductivity is the most important parameter in the design andselection of materials for thermal insulation applications. The axialthermal conductivity of pure NCC and NCC composite foams are reported inFIG. 8 In PVA/NCC samples, the thermal conductivity appeared to berelatively insensitive to the PVA content and it ranged between 0.036and 0.041 Wm⁻¹K⁻¹. However, the thermal conductivity was decreased for10PVA/25BTCA/NCC composite foam with a value of 0.027 Wm⁻¹K⁻¹. Thisaccounts for ˜35% reduction in the thermal conductivity, compared to0.041 Wm⁻¹K⁻¹ of pure NCC and a t-test analysis showed that thedifference between the thermal conductivity of NCC and 10PVA/25BTCA/NCCsamples are statistically significant with 99% confidence interval. Inthe case of NCC composite foams, more than 98% of the volume is made ofair. In such low-density foams, the conduction by gaseous phase becomesone of the major heat transfer mechanisms. It is well established that,at a given relative density, a cellular structure with smaller cellsizes and higher cell density yields better thermal insulationcapability. The smaller the cell sizes are, the more limited is themovement of gas molecules and thus less effective is the heat transferthrough the gaseous phase. Therefore, the smallest cell sizes associatedwith 10PVA/25BTCA/NCC samples resulted in the lowest thermalconductivity. The thermal conductivity achieved here was below the rangefor common commercially available insulation materials, such as expandedpolystyrene (λ=0.030-0.040 Wm⁻¹K⁻¹), mineral wool (λ=0.030-0.040Wm⁻¹K⁻¹, and extruded polystyrene, λ=0.033-0.044 Wm⁻¹K⁻¹). It is alsonoted that the thermal insulation performance of cellulose based foamsmight be affected under hygrothermal exposure and further investigationsare needed in this regard.

3.7. Fire Resistance and Carbonization

The 10PVA/25BTCA/NCC composite foam exhibited a fire resistance behaviorwith low shape distortion and small shrinkage after burning in an openatmospheric condition. The flammability was also further examined byvertical burning test (UL94) of 10PVA/25BTCA/NCC foam. FIG. 9 shows the10P/25BTCA/NCC composite foam before and after 11 s burn, and the foamafter the burning test, indicating that the composite foam maintainedits structural integrity with a minimal shrinkage after burning. Theburning rate of the NCC foam and the composite foam was similar.However, the structure of the pure NCC foam collapsed after burningtestes. It is worth noting that the fire-retardant behavior of thecomposite foams can be further improved by adding inorganic retardantnanomaterials.

The 10PVA/25BTCA/NCC foam was also carbonized to evaluate its stabilityand thermal and mechanical properties after carbonization to assess itspotential as a carbon foam precursor. As shown in FIG. 10A, thecarbonized NCC composite foam shrank in both radial and axialdirections, indicating its relatively isotropic cellular morphology. Thecarbonized composite foam maintained its original porous structure (FIG.10B) but with some level of reduced uniformity compared to the originalcellular structure. This might be related to the uneven local shrinkageof the structure during the carbonization process. The carbonized foamexhibited a relatively low density of 0.018 g·cm⁻³ and a thermalconductivity of 0.065 Wm⁻¹K⁻¹. Based on the compression test, thecarbonized NCC composite foam exhibited a linear elastic behavior at lowstrain levels, and the stress at 50% strain reached to 50 kPa, which isslightly lower than that for the foam sample before carbonization (73kPa). These results show that the cellular structure was kept intactduring carbonization process, indicating a promise of this NCC compositeas a material for superlight carbon foams.

In summary, using a facile freeze-casting method, high-performancecomposite foams based on nanocrystalline cellulose (NCC) with anexcellent combination of thermal insulation capability and mechanicalproperties were developed. An exemplary formulation of 74 wt. % NCC, 7.5wt. % polyvinyl alcohol (PVA), and 18.5 wt. % BTCA/DP resulted incomposite foams that exhibit an elastic strain of ˜13% at a Young'smodulus of 250 kPa and a stress value of 73 kPa at 50% strain; bothexceed the values of reported nanocellulose-based foams withoutreinforcements. The foams exhibit a thermal conductivity of 0.027Wm⁻¹K⁻¹, which is superior to those of traditional insulating materials.

Without wishing to be bound by theory, the chemical interaction betweenthe cellulose nanocrystals and PVA molecules through ester bondsfacilitated by the BTCA crosslinking agent was found responsible for theenhanced mechanical properties; the elasticity and rigidity wereprovided by PVA and NCC, respectively, in a crosslinked integratedNCC/PVA network. The enhanced elasticity and strength also contributedto having a more uniform cellular structure with smaller pore sizesduring the ice crystallization stage, which in turn effectively reducedthe thermal conductivity. While pure NCC foam burnt into ash, theexemplary composite foam maintained its overall structure with someshrinkage after burning.

The exemplary composite foams demonstrate the potential of renewablematerials such as nanocellulose towards high-performance thermalinsulation materials that can contribute to energy savings, less usageof petroleum-based materials, and reduction of adverse environmentalimpacts.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

We claim:
 1. A composite foam, comprising: about 10 wt % to about 95 wt% of a cellulose component comprising about 20% to about 100% cellulosenanomaterial; about 1 wt % to about 25 wt % of a water-soluble polymercomprising a plurality of first crosslinkable groups, about 1 wt % toabout 25 wt % of a water-soluble crosslinking agent comprising aplurality of second crosslinkable groups; wherein at least a portion ofthe second crosslinkable groups forms covalent bonds with at least aportion of the first crosslinkable groups and at least a portion of thehydroxy groups of the cellulose component.
 2. The composite foam ofclaim 1, wherein the composite foam has a porosity of about 30% to about99%.
 3. The composite foam of claim 1, wherein the composite foam has anapparent density of about 0.01 g/cm³ to about 1 g/cm³.
 4. The compositefoam of claim 1, wherein the cellulose component comprises 100%cellulose nanomaterial.
 5. The composite foam of claim 1, wherein thefirst crosslinkable group is a hydroxyl, a thiol, a hydrazide, an aminogroup, or a combination thereof.
 6. The composite foam of claim 1,wherein the second crosslinkable group is a carboxylic acid, ananhydride, an activated ester, an aldehyde, a ketone, or a combinationthereof.
 7. The composite foam of claim 1, wherein water-soluble polymeris polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyacrylic acid(PAA), polyvinyl pyrrolidone (PVP), polyacrylamide (PAM), alginic acid,starch, xanthan gum, dextran, pectin, or a combination thereof.
 8. Thecomposite foam of claim 1, wherein water-soluble crosslinking agent is1,2,3,4-butane tetracarboxylic acid, oxalic acid, succinic acid, malicacid, citric acid, adipic acid, or a combination thereof.
 9. Thecomposite foam of claim 1, wherein the composite foam is formed byfreeze-casting of a precursor composition that does not comprise anorganic solvent.
 10. The composite foam of claim 1, wherein thecomposite foam has a compression stress measured at 50% strain fromabout 50 kPa to about 250 kPa.
 11. The composite foam of claim 1,wherein the composite foam has thermal conductivity from about 0.015Wm⁻¹K⁻¹ to about 0.045 Wm⁻¹K⁻¹.
 12. The composite foam of claim 1,wherein the composite foam has an elastic strain of at least 10% atYoung's modulus of from 50 kPa to about 1500 kPa.
 13. The composite foamof claim 1, wherein the composite foam comprises a colorant.
 14. Thecomposite foam of claim 1, wherein at least a portion of the hydroxygroups of the cellulose component are silanized.
 15. A building productor a consumer product comprising the composite foam of claim
 1. 16. Thebuilding product of claim 15, wherein the building product is aninsulation panel.
 17. The consumer product of claim 15, wherein theconsumer product is a cup, a plate, a shipping container, a cooler, or apotting container.
 18. A method of making a composite foam, comprisingfreeze-casting or freeze-drying an aqueous composition comprising a foamprecursor, wherein the foam precursor comprises: about 10 wt % to about95 wt % of a cellulose component comprising about 20% to about 100%cellulose nanomaterial; about 1 wt % to about 25 wt % of a water-solublepolymer comprising a plurality of first crosslinkable groups, about 1 wt% to about 25 wt % of a water-soluble crosslinking agent comprising aplurality of second crosslinkable groups; wherein at least a portion ofthe second crosslinkable groups can form covalent bonds with at least aportion of the first crosslinkable groups and at least a portion of thehydroxy groups of the cellulose component.
 19. The method of claim 18,wherein the aqueous composition does not include an organic solvent. 20.A composite foam prepared by the method of claim 18.